1. Field of the Invention
This invention relates to voltage reference circuits, and more particularly to voltage reference circuits and methods in which the user can select between a trimmable, internally generated reference voltage, and an externally applied reference voltage.
2. Description of the Prior Art
It is often desirable, for applications such as supplying a reference voltage for an analog-to-digital converter (ADC), to provide a voltage reference circuit which permits the user to select a voltage that is internally generated by the circuit, or to apply a different externally generated voltage through the integrated circuit pins. Also, it is normally desirable to be able to trim the internally generated voltage to ensure that it is within the designed tolerance, since normal fabrication variations and the like will typically result in some of the circuits generating inaccurate voltage levels. Although these needs have been satisfied in the past, it has required the use of three separate pins on the circuit element: one to receive an external voltage source, a second to provide an output for the internally generated voltage, and a third to receive trimming voltages used to trim the internally generated reference. Since pins on an integrated circuit package are frequently at a premium, it would be helpful if these functions could be accomplished with fewer than three pins.